Polyurethane foams, formed by the reaction of a polyisocyanate with a polyhydroxyl-containing compound in the presence of a suitable catalyst, are widely accepted as padding materials for cushions in furniture, automobiles and the like. Polyurethane foams are also used as sponges and for other uses that require liquid absorption properties, such as specialty packaging and personal care and hygiene items.
Polyurethane foam formulations typically require a blowing agent to generate the gas to fill or expand the polyurethane foam cells. The resultant density of a polyurethane foam is controlled by the quantity and efficiency of the blowing agents. While carbon dioxide may be generated as a blowing agent through the reaction of water with the isocyanate, the use of low-boiling inert liquids, in particular chlorofluorocarbons (CFCs), to augment or replace the chemical blowing action has lead to certain property advantages in the final foams, such as low thermal conductivity characteristic of the trapped CFCs.
However, the CFCs used as blowing agents, and for other purposes, are now suspected to be linked to the depletion of ozone in the upper atmosphere where the generally inert CFCs are decomposed by ultraviolet light. To avoid this potential problem, polyurethane foams blown only with carbon dioxide have acquired renewed interest.
The use of carbon dioxide generated from the reaction of water with isocyanate as a sole blowing agent, necessitates formulations containing relatively large amounts of water to obtain low density foam grades, i.e., 5 parts by weight (pbw) water for 1.2 pounds per cubic foot (pcf) density. When the water content exceeds a certain level, about 5.4 pbw, the foams become more difficult to process because of the increased exothermic conditions and resulting threat of scorching and fire.
In addition the major parameters regulating flexible polyurethane foam hardness are the hard segment/soft segment ratio and their distribution in the polymer chain. When water is used as a blowing agent, each part of water added to the foam formulation produces 8.2 parts of high density aromatic hard segment (the polyurea portion). At water levels above 4 parts, this hard segment increase becomes a significant percentage that overpowers the softening characteristics of the standard polyols.
Many attempts have been made to counterbalance the negative influence of the excess urea groups upon the softness of polyurethane foams. For example, one approach is to reduce the polyol functionality while increasing the polymer chain length of the polyol. Polypropylene oxide/polyethylene oxide copolymer triols with molecular weights varying from 2000 to 6000 are blended with diols of similar structure. In high water blown formulations, the softening effect provided by these diol/triol blends is limited if the overall functionality is not greatly reduced and the molecular weight is significantly increased. Such changes in the structure of the polyol blends result in a decrease of the polyol reactivity which has to be compensated by large additions of ethylene oxide. Then, the final polyols become difficult to process and yield foams with poor static fatigue properties and significantly reduced resistance to hydrolysis.
An example of this first approach is seen in U.S. Pat. No. 3,857,800 which describes flexible polyurethane foams made by foaming a reaction mixture containing a polyol, an organic polyisocyanate and a blowing agent, which may be water, which would otherwise produce a closed cell foam which would shrink after its formation wherein the foamable reaction mixture is modified by including therein a subsidiary polyol which is different from the primary polyol and which has a molecular weight of about 500 to 3500 and contains at least 40% by weight oxyethylene groups with at least some of them in a non-terminal position, the amount of subsidiary polyol being not more than 50% by weight of the total polyol in the reaction mixture. The subsidiary polyol may be a polyethylene ether glycol.
Related to this patent is U.S. Pat. No. 3,943,075 which discusses flame-resistant polyurethane foams obtained by reacting a tolylene diisocyanate (TDI) with a polyol in the presence of a substance which is normally effective for the polymerization of TDI and an anti-aging additive such as a halogenated aliphatic phosphate. A subsidiary polyol such as the one of the '800 patent may also be present. Companion U.S. Pat. No. 3,803,064 covers the processes for making the foams of the '075 patent. French Patent 2,095,362 is also related to this group of patents.
Additionally of interest with respect to this approach is U.S. Pat. No. 4,259,452. It relates to a method of producing flexible polyether polyurethane foams, which have a substantially skeletal reticulated structure. When the flexible foam is produced by reacting a polyhydroxyl compound with an organic polyisocyanate in the presence of a blowing agent by a one-shot process, a catalyst and other additives, a mixture of (a) poly(oxyethylene-oxypropylene)polyether polyol containing 51 to 98% by weight of ethylene oxide component and (b) poly(oxyethylene-oxypropylene)polyether polyol containing not less than 51% by weight of propylene oxide component is used as the polyhydroxyl compound.
A second approach is to reduce the excess urea groups by decreasing the isocyanate index of the all water blown formulations. With this method, using conventional poly propylene/poly ethylene oxide copolymer triols of molecular weight varying from 2000 to 6000, both the foaming process and the foam physical properties deteriorate rapidly when the isocyanate index drops below 103.
The low crosslink density of the foam at the early stage of polymerization yields weak cell structure and partial collapses. Increasing the catalyst and surfactant levels overcomes this lack of stability and yields a foam with closed cell structure and potential shrinkage. As a result, some of the major foam physical properties such as the tear resistance, the tensile, the elongation, are completely deteriorated, while the static fatigue values become marginal.
However, since neither of these approaches has proven to be satisfactory, it would be useful to devise an improved polyurethane composition employing only water as a blowing agent which would not have its properties degraded at all, and particularly in the manner of the prior approaches.